1. Field of the Invention
The present invention relates to a method and apparatus for analytical determination of components in fluid mixtures by employing diffusion of the component to be determined through a membrane.
2. Brief Description of the Background of the Invention Including Prior Art
Blood contains dissolved oxygen (O.sub.2) and dissolved carbon dioxide (CO.sub.2). The partial pressures, or tensions, are important diagnostic parameters and are frequently measured in clinical settings. Most conventional methods perform such determination in vitro and blood is taken anaerobically via blood vessel puncture and then measured.
Brantigan et al. in Journal of Applied Physiology, Vol. 40, No. 3, March 1976, page 443 show a clinical catheter for continuous blood gas measurements by mass spectrometry. Massaro et al. in Biomat. Med. Dev. Art. Org., Vol. 4, Pages 385-396 (1976) teaches a system for diffusing dissolved blood gases into the lumen of a close-tipped indwelling catheter to approach equilibrium concentration in a carrier gas. Huchon et al. in Europ. Journal Intensive Care Medicine Vol. 2, 23-28 (1976) employs catheter electrodes for determining in vivo the partial pressures of oxygen and carbon dioxide respectively. However, the electrodes as employed could be influenced by other components contained in the blood. Niinikoski et al. in Adv. Exp. Med. Biol. vol. 94, pages 582-592 (1977) disclose implanting a silastic tube between muscles and allowing oxygen to equilibrate through the permeable silastic tube into a hypoxic saline solution. J. S. Clark et al. in Adv. Exp. Med. Biol. Vol 94, pages 31-36 (1977) teach the employing of an in vivo tube containing liquid and coming to equilibrium with body fluid gases.
Thus there are known several methods for determining gas components in blood employing the reaching of equilibrium condition between a body fluid and an external fluid through a membrane followed by an analytical determination of the concentration of the gas component in the external fluid. Such methods are uncertain as it is difficult to assure that equilibrium conditions have been reached and they are furthermore quite time-consuming.
Thus at best conventional systems measure the rate at which blood gases diffuse through a permeable membrane. In general, the correlation of this rate of diffusion with the partial pressure of the gases in the blood is established by calibration of each individual pick-up prior to insertion into the blood stream. Changes in gas transmission of the membrane for example due to deposition of blood components will give erroneous blood gas partial pressure measurements. Thus an unpredictable change in the diffusion rate of the gas through the membrane in a conventional determination would result in errors in the partial pressure determination of the component. This error is independent of the specific analytical determination method applied to the test fluid such as detector mass spectrometry, simple manometry or minaturized polarographic detector mounted on the tip of a catheter.